One of the problems encountered in development of optical communication systems is the necessity to create uniform optical intensity distribution across the beam transmitted along the communication path, as well as uniform distribution of optical radiation power among the components of the beam angular spectrum (see, for example, U.S. Pat. No. 5,892,866 to Bristow et al.).
If a multimode optical fiber coupled to a telescope is used in the output beam shaping device, the above problem may be solved by transformation of the initial beam from a radiation source into a multitude of fiber modes with a uniform distribution of radiation power between the modes, by means of various technical solutions.
Uniform distribution of power between modes provides average uniformity of intensity distribution in the beam cross-section and over its angular spectrum. It should be noted however, that when modes are excited in a multimode optical fiber, a fine-structure nonuniformity of intensity distribution across the fiber is observed, which is caused by inter-mode interference (what is known as “speckles”).
Speckles can not be avoided by uniform excitation of fiber modes. But if speckles in a receive aperture plane of a remote receive terminal could be made small enough compared to the aperture, it is sufficient to achieve average (averaged over speckles) uniformity of the intensity distribution to ensure stable operation of a communication system. Prior art techniques, however, have been ineffective and/or impractical for uniform excitation of fiber modes as well as for averaging speckles out by the receive aperture, particularly for use in free-space optical communications.
The intent of this patent application is to provide average uniformity of the intensity distribution in a receive aperture plane as well as to eliminate influence of speckles on optical communication system operation.